1. Field of the Invention
The present invention relates to a wireless communication device preferably for use in an RFID (Radio Frequency Identification) system or a near field communication (NFC) system that communicates with a partner device via an electromagnetic field signal.
2. Description of the Related Art
NFC is a wireless communication standard combining contactless identification technology and interconnection technology. Devices conforming to this standard use a frequency in the 13.56 MHz band and perform bidirectional communication at a short distance of about a few tens of cm. There are multiple communication speeds (106 kbps, 212 kbps, 424 kbps, and 848 kbps) in NFC systems. Therefore, a wide-band antenna is necessary in order to perform communication without waveform distortion even at the highest communication speed.
A design technique for widening the band of an antenna includes, as described in Japanese Unexamined Patent Application Publication No. 2010-109806 and Japanese Patent No. 4645762, a technique that provides multiple resonant circuits close to one another, thereby performing multiple resonances and widening the band.
As in Japanese Unexamined Patent Application Publication No. 2010-109806 and Japanese Patent No. 4645762, in the technique which provides multiple resonant circuits close to one another, thereby performing multiple resonances, it is essential to form multiple LC resonant circuits, and it is thus necessary to secure sufficient spaced for the multiple LC resonant circuits. Thus, it is difficult to embed the LC resonant circuits in a small wireless communication device such as a mobile terminal.
In addition, if an antenna with wide-band characteristics is simply used, the antenna efficiency cannot be efficiently used at the time of low-speed communication, and it is impossible to secure a necessary communication distance with a small antenna. The Q value of an antenna is expressed as “Q=fo/B.W.” (where B.W. is the bandwidth of the antenna, and Fo is the center frequency). In general, Q deteriorates as the bandwidth of an antenna becomes wider. That is, in the case of using an antenna with wide-band characteristics, the antenna is suitable for high-speed communication since the antenna itself is designed to be wide-band. However, at the time of low-speed communication where no problem is caused even when the bandwidth is narrow, the antenna is used in a state where the Q value is poor.